Relay

Relay is a WSL2 Linux process created by a session leader. Its job is to create a Linux process on behalf of the user, and relay its output back to Windows.

Creating a user process

A relay is created when a LxInitMessageCreateProcessUtilityVm message is sent to a session leader. Once created, the relay creates multiple hvsocket channels with wslservice.exe.

These channels are used to:

• Relay standard file descriptors (stdin, stdout, stderr)
• Relay information about the terminal (for instance when the terminal window is resized from Windows)
• Notify Windows when the Linux process exits

Once those channels are configured, the relay forks() into two processes:

• The parent, which will read & write to the child's standard file descriptors and relay it to Windows
• The child, which calls exec() and starts the user process